This paper reports on two experiments conducted in an attempt to extend findings by Ellis which suggest a rehearsal deficit in mentally retarded subjects. In experiment one, mentally retarded subjects saw nine stimuli in a serial position probe task for either two, four, or six seconds each. Performances for the two and four second-per-item rates of presentation did not differ and were similar to those obtained by Ellis for rates of presentation of two seconds or less. However, when stimuli were shown for six seconds each, performance was facilitated. In experiment two, seven stimuli were presented at rates of four, six, or eight seconds per item. During this interval stimuli were exposed for either two seconds or for the entire interval. The eight-second presentation rate was superior for both exposure durations to the four- and six-second rates, which did not differ from each other. (Author/WR)